The art of making bladeholders for doctors and scrapers is old, yet continues to develop. The task of doctoring or scraping a moving work surface, as on a roll or cylinder, for example, present problems of approach to the load, operational control, and blade wear and replacement, which continue to engage paper makers and others facing the task. In the paper industry, doctors and scrapers are employed to clean the surfaces of rotating calendar rolls, drier cylinders and the like; and scrapers are used to remove a web of paper as in the manufacture of crepe paper (creping doctor). Scrapers (sometimes called "knives") are used to remove product from drums in flakers and drum driers used to prepare dried products of various kinds (example being food stuffs, pharmaceuticals, chemicals, films solidified from liquids) from a starting liquid or paste.
When the surface to be doctored or scraped (working surface) is that of a material soft enough to be damaged by a blade or knife approaching it at an angle that favors digging in, chipping or otherwise injuring the working surface, it is important that the bladeholder control not only the angle at which the blade engages the work surface during operation, but also the angle at which the blade approaches the work surface when being brought into position for operation.
Blades wear out in use, and many attempts have been made to extend blade life. Attention has been given to special treatments of the working edges of blades, to improve the lasting qualities and efficiencies of the working edges. Attention has also been given to making blades having reserve body structure and adjustable holding features such that, in combination with a bladeholder incorporating cooperating adjusting features, the blade can be adjusted to compensate for wearing away of its working edge.
Much attention has been given to problems of operational control. Doctor and scraper blades are generally long, thin structures as much as one to six inches wide and extending sometimes as much as 35 feet or more from one end to the other, across a working surface transversely to the direction of relative motion of the working surface. In paper machinery, the long dimension of the blade is in the "cross-machine direction" (CMD). Obviously, the blade is subject to flexure in the CMD, and a high spot on the working surface, due to debris, for example, can lift the blade from the working surface in the vicinity of the high spot, and create conditions of non-uniform pressure between the blade working edge and the working surface in the CMD. This is a transitory, or dynamic flexure problem. There exists also a static problem of blade flexure to accommodate roll crown and the like.
It is known in Scallen's U.S. Pat. No. 488,455, and Harvey U.S. Pat. Nos. 429,381 and 481,866 that non-metallic scraper blades may be mounted substantially perpendicular to a roll in a roller mill and used (in Harvey) to prevent the accumulation of crushed grain on the roll in the manufacture of flour. The blades are made of leather, wood, paper board, or hard rubber, and a purpose is stated by Harvey to be to remove danger of fire or abrasion of rolls. Scallen teaches adjustment to contact the roll. In Vickery U.S. Pat. No. 1,883,167 a doctor for paper making machinery is shown with a blade positioned at an angle about 45.degree. or less to the roll and held by a rigid support with the blade pressed against the roll by spring means bearing on said blade. Vickery U.S. Pat. No. 1,945,761 shows a curved blade having a rear edge clamped to a rigid carrier and its consumable forward part adapted to bear edgewise on a roll. LaFore U.S. Pat. No. 1,845,716 shows an early example of a doctor blade with thin slots extending inward from its rear edge, in a so-called "floating" mount.
Holcomb U.S. Pat. No. 2,330,889 describes a paper making roll doctor having a blade support and blade loading means employing a plurality of separate pressure fingers, operable against a side of the blade, which are individually controlled to force the blade into resilient contact with the roll. Actuable means are described to control the movement of pressure fingers against a side of the doctor blade.
In Miller, U.S. Pat. No. 2,915,421 a straight blade mounted to engage a roller radially along its center line, primarily for cleaning purposes, is shown with spring means at the back edge of the blade to press radially inwardly on said blade for forcing the front edge of the blade against said roller.
Generally, representative prior solutions to blade flexure and roll-contact problems are described in Ljungquist U.S. Pat. No. 2,477,339; DST Pattern and Engineering U.S. Pat. No.'s 3,163,878 (Smith et al) and 3,529,315; (Dunlap et al); and Goodnow 3,778,861.
Despite the fact that the art of paper making is by now quite old, the manufacture of crepe paper with the aid of a creping doctor presents problems which affect the quality and quantity of finished product. Present creping doctors used in the industry require relatively frequent blade changes due to the type of wear on the blade edge in contact with a cylindrical drier or the like. As can be seen in FIG. 1 at position A, a conventional creping doctor blade 10' is typically positioned with one side of its front edge against a cylindrical roll, such as a Yankee dryer, and as the roll moves in the direction of the arrow 30, the blade will wear away the front edge 1 and eventually take on a knife-edge shape as shown in FIG. 1, position B. Due to this type of blade wear, the quality of a product manufactured may progressively change, and with it the operational efficiency.
The art of creping paper, food stuffs or some other material requires the accurate, uninterrupted, and uniform removal of the web material from its carrying surface, normally a cylindrical surface. In order to do so the blade must be positioned so that it presents a creping surface making an acute angle to the oncoming direction of the material web. As is shown in FIG. 1 at position B, a blade in the conventional position wears so that the creping surface 1 of the blade decreases in thickness until it is not sufficiently wide for creping purposes. In this condition of wear, the blade is acting as a scraper and thus should be replaced. As wear progresses from the initial condition, as shown in FIG. 1 at position A, to conditions approaching that of FIG. 1 at position B, the worn "heel" or width of surface contacting the rotating drum substantially increases, thus reducing the unit loading (PSI) between these surfaces. When the blade has worn to a state as shown in FIG. 1 at position B, there exists a tendency for the leading edge, point 1, to lift or curl away from the roll. This lifting associated with the leading edge is due to the thinned condition of metal at the leading edge and its exposure to the heat generated by the friction between blade and roll. Reduced unit loading and edge curl enhances the possibility of paper fibers lodging under the crepe blade, a condition called picking, resulting in disruptions of the uniformity of the creped product. This wear characteristic of conventional creping doctors require frequent blade changes resulting in substantial machine down-time for industry. Considering the fact that modern paper machines are operated at high speeds, generally at two to four thousand feet/minute, machine down-time is a significant problem for industry.
Conventional creping doctors cause a significant amount of friction between blade and roll. This is due to the fact that the total applied load force on the working edge of the blade is high in order that the creping doctor maintain a suitable creping position for the narrow blade edge. The presence of high friction substantially increases the energy required to rotate the cylindrical dryer or roll. In today's age of expensive energy sources, the increased energy requirements present severe economical drawbacks to the present mode of creping.
Additionally, the heat caused by the friction between blade and roll causes the relatively long, (normally as much as 30 feet) blade to ripple at the working edge, resulting in an uneven contact between blade and roll. With an uneven contact between blade and roll, fiber will tend to slide under that portion of the blade that is lifted off the roll, thus aggravating the problem of edge ripple.